*   Indenting error output cells
*   Removing explicit 'level' keys from Markdown files
*   Storing the generated files so that people who don't use those tools won't have to regenerate them.
*   Modifying .gitignore to reflect this.
*   Updating the Makefile to run Jekyll exactly once.
*   Command to install on the server
*   No longer worrying about making a page of images
*   Better (more guessable) name for the target that builds the website
*   Fixing image paths
*   Fixing up glossary entries

**Note**: This is a prototype.

Instead of "In" and "Out" use arrow to mark the input and output
of IPython lessons.

To construct the arrows we only use CSS.